From the electrothermal reduction furnace for producing yellow phosphorus is withdrawn a dust-containing gas mixture essentially composed of vaporous phosphorus and carbon monoxide which, prior to the condensation of the phosphorus contained therein, passes through an electrostatic precipitator in which the dusts are retained. These dusts which contain about 20% by weight of P.sub.2 O.sub.5, as further major constituents SiO.sub.2 and CaO and as minor components about 1.0 to 1.5% by weight of lead, about 0.3 to 0.5% by weight of cadmium and 5 to 6% by weight of zinc, are agitated with water to form a dust mash. From this dust mash, together with clay mash as binder, moldings can be prepared from crude phosphate on a granulation device which are then fired on a sintering device to form phosphate pellets.
In U.S. Pat. No. 4,537,756 it is proposed, for the removal of zinc, cadmium and other heavy metals, to process ground phosphorus furnace slags together with dust mash on a granulation device to form moldings, where the moldings, after drying and calcination, form pellets which contain the metals mentioned and which can be placed in a landfill.
A disadvantage, in particular, in the last-mentioned process is that zinc, cadmium and the other heavy metals contained in the pellets cannot by this means be taken to any further utilisation. Occasionally the dusts show radioactivity originating from the crude phosphate (lead 210 and derivatives).